This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from my application CIRCUIT FOR STABILIZING A SCREEN VOLTAGE OF A VIDEO DISPLAY APPARATUS filed with the Korean Industrial Property Office on Sep. 10, 1998 and there duly assigned Ser. No. 37392/1998.
1. Field of the Invention
The present invention relates to stabilizing a screen voltage of a video display apparatus, and more particularly, to a circuit for stabilizing a screen voltage of the video display apparatus capable of supplying a uniform screen voltage to the video display apparatus even though a voltage level of an output voltage from a flyback transformer is changed.
2. Description of the Related Art
In general, a video display apparatus such as a monitor of a computer system, responsive to a sync signal, displays an externally input video signal on a screen by colliding an electron beam corresponding to the video signal with luminescent materials arranged on a fluorescent screen of a cathode-ray tube.
To display the video signal on the screen as described above, the video display apparatus has to sequentially execute operations which control, accelerate and focus the electron beam corresponding to the video signal with a plurality of grid electrodes arranged in the cathode-ray tube.
The plurality of grid electrodes includes a first grid electrode for controlling an emission amount of an electron beam, a second grid electrode for controlling a moving velocity of the electron beam, a third grid electrode for focusing the electron beam, and a fourth grid electrode for controlling the diffusion of the electron beam according to the movement of the electron beam.
A voltage supplied to the second, third and fourth grid electrodes is generated by a flyback transformer. Accordingly, the brightness and quality of the screen are determined by the stability of the voltage which is respectively supplied to the second, third and fourth grid electrodes by the flyback transformer.
To stabilize the voltage which is respectively supplied to the second, third and fourth grid electrodes, a peak value of a flyback pulse inputted to a primary winding of the flyback transformer has to be maintained at a uniform voltage level.
However, the peak value of the flyback pulse is often changed according to the change of a load of a voltage induced in a secondary winding of the flyback transformer. Consequently, a level of the voltage induced in the secondary winding of the flyback transformer is changed.
Accordingly, since the voltage which is respectively supplied to the second, third and fourth grid electrodes by the flyback transformerxe2x80x94particularly, a voltage level of a screen voltage supplied to the second grid electrodexe2x80x94is changed, the quality of the screen of the video display apparatus deteriorates.
Therefore, it is an object of the present invention to provide a circuit for stabilizing a screen voltage of a video display apparatus capable of supplying a uniform screen voltage to the video display apparatus even though a voltage level of an output voltage from a flyback transformer is changed.
In order to achieve the above object, the present invention provides a circuit for stabilizing a screen voltage of a video display apparatus, which comprises: a horizontal output generator for generating a horizontal output voltage in response to a horizontal driving signal; a flyback transformer for supplying a first power source to a primary winding according to the horizontal driving signal, for inducing a voltage in a secondary winding thereof based on a turns-ratio, and for inducing a voltage in a tertiary winding thereof according to the voltage induced in the secondary winding based on the turns-ratio; and a screen voltage generator for receiving a second power source and a pulse width modulation signal, for controlling the voltage induced in the tertiary winding of said flyback transformer to maintain a uniform voltage level by selectively outputting the second power source in response to the pulse width modulation signal, and for outputting the voltage induced in the tertiary winding as a screen voltage.
According to the present invention, the screen voltage supplied to the second grid electrode from the flyback transformer is generated by the voltage induced in the tertiary winding thereof, Also, the voltage induced in the tertiary winding of the flyback transformer maintains a uniform voltage level by controlling the screen voltage generator even though the high voltage and the voltage induced in the tertiary winding are changed. Accordingly, the invention is able to prevent the brightness and quality of the screen of the video display apparatus from deteriorating.